Hi,
I've had two 28-135. I let the first one go when I upgraded to 24-70/2.8L, but then ended up buying another as a backup.
I like the focal length range a lot, on full frame or crop sensor cams, but I also have a 12-24/4 Tokina for use on my crop sensor cameras and 24mm, 20mm lenses for use on full frame. So I don't feel the lack of wide angle with it.
The 28-135 has a lot going for it, especially considering large numbers of them have been sold in kit and it's pretty easy to find a good, lightly used copy for around $250. It normally sells for over $400, if bought new (Canon's MSRP is $480).
USM gives you fairly fast, quiet focusing, as well as Full Time Manual (with non-USM lenses you need to turn off AF at the switch before manually overriding AF, or you will break the focus system.. with USM like the 28-135 you can simply override AF manually any time you like and don't need to worry about turing off the switch).
It's pretty close focusing, too... I think to about 20 inches (closer is possible with macro extension tubes behind it). And it's got decent Image Stabilization, useful out at the 135mm end of things in particular, although it's one of the simpler versions of IS that's good for probably two stops assistance and you are supposed to turn it off manually when using the lens locked down on a tripod. Build quality is decent, typical Canon "mid-grade" (similar to other USM lenses, such as 50/1.4, 85/1.8, 28/1.8 primes that I use personally). It's not an "L" build, has more plastic than that. But not nearly as plasticky as the entry-level lenses (18-55, 50/1.8, 55-250, etc.)
I highly recommend getting the accessory lens hood for it (one was included with the used lens I bought, as well as a B+W MRC UV filter that probably sells for $60-70). The Canon hood will cost about $25-30. You might find cloned third party hoods on certain auction sites for less. The lens uses 72mm filters. (Note: IMO, it's a waste of money possible risk to image quality to put a "protection" filter on any lens 24/7/365. I use 'em, but only when really needed, say in a sand storm.)
Yes, most copies of the 28-135 show that pesky "zoom creep". It's a small nuisance, IMO. And as an f3.5 to f5.6 variable aperture zoom lens, it's not particularly fast. Decent background blur (bokeh) is possible, just not as much or as smooth as possible as with larger aperture and much more expensive lenses that have more and/or specially shaped aperture blades (28-135 - 6-blade, standard; 24-70/2.8L - 8-blade, curved; 24-105/4 IS - 8-blade, curved... more blades and particularly curved ones give a more perfectly round aperture, which makes for smooter background blur.)
Ultimately, image quality is the most important consideration with any lens. For most typical purposes, moderate size enlargements, I have to say the 28-135 works quite well. I cannot really tell much difference between it's images and those made by 24-70, unless a particularly large aperture were used with the L lens. I actually think the 28-135 exhibits less optical vignetting than the 24-105, at some focal lengths.
It's an EF lens that's usable on both full frame and crop sensor cameras (unlike EF-S which are only usable on 1.6X cameras). On a crop sensor camera and in combination with an UWA lens such as the Tokina 12-24 I use personally, the 28-135 gives you a nice, solid, two lens kit that actually covers a wider range of focal lengths than most 35mm film shooters ever owned in their lifetime. (Nearly all had a 50mm that came with their SLR, a lot would add a 70-210 or similar, some would also get a 28mm as a wide lens... While two lenses covering 12mm to 135mm used on 1.6X camera equate to "19mm to 216mm" range.)
Finally, and some budget-conscious pros use the 28-135. For example, Joe Farace at Shutterbug Magazine is a big fan of the lens. You'll often see his Canon camera tests, studio and location work using this lens. He's said he thinks it silly to spend $1000 plus for a "pro" lens when there is a far less expensive lens such as this available that's so darned good. Obviously I don't completely agree, since I use 24-70/2.8 primarily and keep the 28-135 as a backup or a lens to put on a spare camera and hand to an assistant to use.